1. Technical Field
The present invention is directed toward machines for molding objects, and more particularly toward carriages for use in positioning the mold blocks in such machines.
2. Background Art
A well known method for manufacturing corrugated pipes is to extrude a tube of thermoplastic from a head and then conform that tube to the interior of a corrugated mold tunnel formed by mold blocks. The mold blocks typically come in pairs and mate together to form a portion of the mold tunnel, and the thermoplastic is either blown into its shape (by pressure created within the mold tunnel) or by vacuum (where vacuum draws the air from around the mold tunnel).
Structures which have been used in the vacuum method of manufacture are illustrated in U.S. Pat. Nos. 4,318,872, 4,439,130, and 4,718,844. These patents all address the problem of drawing a uniform, strong vacuum around the entirety of the mold blocks which are continually moving during the molding process.
Moving and properly aligning multiple mold blocks is important to ensure that the formed product is continuously uniform along its length without distortions which could occur if the mold blocks are not properly aligned relative to each other. In clamshell type machines, for example (with the mold blocks supported on a single carriage on one side), any forces incurred on the other side of the mold blocks naturally involves a larger moment arm about the carriages, with the result being that such structures can be more susceptible to xe2x80x9csawtoothxe2x80x9d type arrangements through the mold tunnel, with successive mold blocks being tilted slightly rather than perfectly aligned with the axis of the mold tunnel.
In order to properly align the mold blocks, it has been found to be advantageous to carry the mold blocks on carriers which themselves have wheels which ride in a track about the path desired for the mold blocks. In order to secure the carriages in the track, the wheels must generally be trapped by opposite sides of the track. In such cases, clearance must be provided (i.e., the opposite track sides must be spaced apart a distance which is greater than the wheel diameters) to allow the wheels to roll on one side without dragging on the other. Such clearance can, however, result in some looseness of the carriage, thereby allowing some degree of sawtoothing. One structure which has been used to address this is shown in U.S. Pat. No. 5,489,201, in which an extra roller is provided along the sides of the carriages, such extra wheel being offset from the other carriage wheels so that it will roll along the top of the machine track while the other carriage wheels roll along the bottom of the machine track.
Rollers have also been provided on carriages and similarly guided in tracks to secure the carriages against lateral shifting during motion. U.S. Pat. No. 4,439,130 shows such a structure. The clearance which must be provided with those rollers (again to prevent the wheel from dragging on one side of the track while rolling on the other side) does, however, potentially allow some degree of undesirable lateral shifting of the carriage and carried mold blocks.
The present invention is directed toward overcoming one or more of the problems set forth above.
In one aspect of the present invention, a molding machine is provided including a plurality of carriages for carrying mold blocks in an axial direction. The carriages each include wheels on opposite sides which follow a guide track along an endless path including a molding section extending in the axial direction. Two wings are secured to the carriages for pivoting about an axis extending in the axial direction whereby in one pivotal position the mold blocks are together in a molding position and in another pivotal position the mold blocks are separated in a releasing position. Each of the wings is adapted to carry one of the mold blocks and has a flat axially extending surface and guide wheels. A wing guide track engages the wing guide wheels and is adapted to place the wings in the molding position when the carriage is in the molding section. A first set of track rollers along the molding section engages the flat axially extending surface of one of the wings of carriages in the molding section along a first rolling plane extending in the axial direction. A second set of track rollers along the molding section engages the flat axially extending surface of the other of the wings of carriages in the molding section along a second rolling plane extending in the axial direction.
It is an object of the present invention to provide a molding machine which may be reliably operated.
It is also an object of the present invention to provide a corrugator for making plastic tubing which may be reliably operated.
It is another object of the present invention to provide a corrugator which will reliably operate to make tubing of various diameters, including large diameter tubing.
It is still another object of the present invention to provide a corrugator which will operate as above at minimum cost.
Another object of the present invention is to provide a corrugator which may be operated at relatively high speeds over long periods of time without degrading the quality of the produced tubing.